Hi Fred, I divide categorical approaches to SQL into two groups. The first are 'programming language theory' centric approaches, where you translate SQL into a lambda calculus containing either monad comprehensions, folds/structural recursion, or a combination of both. This 'mainstream, traditional' theory provides the foundation for most 'language integration query' systems today (e.g., MS LINQ). A good overview is my PhD thesis: http://wisnesky.net/dissertation.pdf . The other group is 'everything else', and there are many one-off approaches and some large bodies of work. Of particular interest are the papers centered around Rosebrugh and Wood, e.g., http://www.mta.ca/~rrosebru/articles/rdic.pdf .